Preparation and Characterization of Silver Orthophosphate

www.nature.com/scientificreports OPEN Preparation and characterization of silver orthophosphate photocatalytic coating on glass substrate Masahide Hagiri*, Kenichi Uchida, Mika Kamo Sasaki & Shofyah Sakinah The photocatalytic activity of silver orthophosphate Ag3PO4 has been studied and shown to have a high photo-oxidation capability. However, there is few reported example of a simple method to prepare Ag3PO4 coatings on various substrates. In this study a novel and simple method to immobilize A g3PO4 on the surface of glass substrates has been developed. A silver phosphate paste based on a polyelectrolyte solution was applied to a smooth glass surface. The resulting dried material was calcined to obtain a coating that remained on the glass substrate. The coating layer was characterized by X-ray difraction and energy dispersive X-ray spectrometry, and the optical band gap of the material was determined. The results indicated that an Ag3PO4 coating responsive to visible light was successfully prepared. The coating, under visible light irradiation, has the ability to decompose methylene blue. Although the coating contained some elemental silver, this did not adversely afect the optical band gap or the photocatalytic ability. Te development of sustainable energy utilization and environmental purifcation technologies is an important issue for ongoing development of society. In this context, research on photocatalysts is being actively conducted. Titanium dioxide TiO 2, a typical inorganic solid photocatalyst, is widely used due to its excellent photocatalytic activity and ability to produce hydrogen by water splitting1–5. However, the development of photocatalysts that utilize visible light, which accounts for 43% of sunlight, has been studied as an efective means of solar energy use3,4. Photocatalysts driven by visible light are also important in the utilization of indoor light as an energy source6. Recently, the photocatalytic activity of silver orthophosphate, Ag 3PO4, has been investigated, and the com- pound has been shown to have a high photo-oxidation capability 7–12. Photo-oxidative decomposition experiments using methylene blue revealed good photo-oxidative performance with a quantum yield of nearly 80%, which is tens of times more efcient than titanium dioxide or bismuth vanadate. In the photo-oxidative decomposition of water to generate oxygen, the performance of Ag3PO4 surpassed that of bismuth vanadate BiVO4 and tungsten 7 oxide WO3 under visible light . Although silver phosphate cannot be used for the conversion of water to hydrogen due to its slightly low conduction band potential, it has shown considerable promise for oxygen generation, decomposition of organic matter, and in antifouling applications8. In addition, recent studies have revealed the antimicrobial properties of materials containing silver phosphate 13. For this reason, there have been numerous 7 reports on the synthesis and utilization of Ag3PO4, especially since the report of Yi et al. Silver orthophosphate may be synthesized by precipitation 14–16, ion exchange15,17, electrolysis7, and other methods 18,19, each of which has its own advantages. Recently, silver phosphate crystals of various shapes15,17–20, core–shell particles21,22, and composites, all of which provide high photocatalytic activity, have been studied 10,12,23–30. Te development of coating technologies for substrates is an important issue when using photocatalysts for surface antifouling applications 31. Tin-flm and thick-flm technologies for semiconductors are also important in the development and advancement of electronic devices 32,33. Te immobilization of titanium dioxide, a typical photocatalyst, on the surface of materials has been the subject of much research and has been widely used in practical applications5,34–37. Tere are various ways for producing semiconductor flms, including the sol–gel method, vacuum deposition, sputtering, plasma CVD, and pulsed laser deposition. In the case of Ag 3PO4, it is difcult to use an approach Department of Applied Chemistry and Biochemistry, Fukushima College, National Institute of Technology, Nagao 30, Kamiarakawa, Taira, Iwaki, Fukushima 970-8034, Japan. *email: [email protected] Scientifc Reports | (2021) 11:13968 | https://doi.org/10.1038/s41598-021-93352-z 1 Vol.:(0123456789) www.nature.com/scientificreports/ such as the sol–gel method to synthesize a coating flm directly from a precursor solution on a solid surface, or a deposition method using plasma. 10 Noureen et al. investigated the antibacterial and photocatalytic properties of Ag3PO4/graphene oxide coated 38 on cotton textiles. For these purposes, the coating has exhibited high performance. Furthermore, Xie et al. prepared titanium plates coated with polydopamine, graphene oxide, and Ag 3PO4 and successfully eliminated bioflm on the metal surface. Nevertheless, these materials were coatings in which crystals were electrostatically deposited on a portion of the substrate surface. An example of fabrication for an Ag 3PO4 coating that adheres to substrates is electrolytic deposition of silver plate, as reported by Yi et al.7. However, the substrate in this method is limited to a conductive medium such as silver plate. Te development of a simple method for the preparation of Ag3PO4 coatings on a variety of substrates may lead to more widespread use of Ag3PO4 as a highly efcient photocatalyst. In particular, if a method can be established for forming flms on transparent materials such as glass, it will be possible to apply Ag3PO4 to optical devices and photoelectrodes, and this should contribute greatly to the development of optoelectronics using Ag3PO4. Tere are few reports for Ag3PO4 coating on transparent substrates. For example, Ma et al. successfully 39 40 immobilized an Ag/Ag2O/Ag3PO4/Bi2WO6 photocatalyst on a glass surface . Gunjakar et al. deposited Ag 3PO4 on the surface of ITO substrate by chemical bath deposition method. For the utilization of Ag 3PO4, it is desirable to discover more versatile coating methods. In this study, we have developed a novel and simple method to immobilize Ag 3PO4 as a coating on the surface of a glass substrate. Te approach is based on a simple method for preparing titanium dioxide coatings for dye-sensitized solar cells via a precursor paste, but the composition of the paste is even simpler in the present application. Tis article is based on a study frst reported in a short communication (in Japanese)41, to which substantial discussion and additions have been made. In this paper, the coating layer was obtained by frst preparing a paste, applying it to a glass surface, and then calcination. Te obtained coating layer was subjected to scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), and X-ray difraction (XRD) analyses. Also, the optical response of the samples was evaluated by measuring their difuse refection absorption spectra. Termogravimetry (TG) and diferential thermal analysis (DTA) were also performed to study the thermal reactivity of the dried paste. Materials and methods Materials. All the water used in the experiments was distilled once and then purifed by ion exchange. Sil- ver nitrate (Wako Pure Chemicals), disodium hydrogen phosphate (Kanto Chemical), carboxymethyl cellu- lose sodium salt (CMC-Na; Wako Pure Chemicals), and methylene blue (Wako Pure Chemicals) were used as received without purifcation. Te commercial silver phosphate (Sigma-Aldrich), silver (Sigma-Aldrich), barium sulfate (Wako Pure Chemicals) were also used without further purifcation. Silver orthophosphate was synthesized according to the method for producing silver phosphate fne particles reported by Khan et al.16. Specifcally, 100 mL each of 0.020 mol L−1 aqueous silver nitrate solution and 0.020 mol L−1 aqueous disodium hydrogen phosphate solution were dropped simultaneously into 200 mL of stirred pure water. To remove the supernatant and obtain fne crystals, the resulting colloidal solution of Ag3PO4 was centrifuged. Tis procedure was repeated at least three times, until no precipitation of silver chloride occurred, whereupon sodium chloride solution was added to the supernatant. Te resulting precipitate was dried under reduced pressure to obtain fne particles of Ag3PO4. Te resulting powder was yellow in color. To study the efect of the dropping rate on the product, the synthesis was attempted with two diferent dropping rates. Te XRD patterns and SEM images for the obtained samples were measured. Te XRD patterns for the samples synthesized at dropping rates of 0.020 cm3 s−1 and 0.33 cm3 s−1 are shown in Fig. 1. For comparison purposes, the difraction pattern for commercial silver phosphate (Aldrich, > 99%) is also shown. Te samples obtained using the above method showed prominent difraction peaks at 33.3° (210), 36.6° (211), 52.7° (222), 57.3° (321), and 55.0° (320)22. All of these major peaks were consistent with the commercial sample and assigned to the body-centered cubic structure of silver phosphate (JCPDS, card No. 6-505). Te results indicated that the synthetic products were Ag3PO4. A slight overlap of the halo pattern indicated the presence of small amounts of amorphous or low-crystallinity product. Te SEM images of the products obtained for each dropping rate are shown in Fig. 2. Te crystal shape is hexagonal prismatic, which is consistent with the crystal structure of silver phosphate. Te dropping rates of 0.33 cm3 s−1 and 0.020 cm3 s−1 yielded relatively uniform crystals of size ca. ~ 500 nm and ~ 1 µm, respectively. Te particle size is smaller for synthesis at the increased dropping rate. Te rapid dropping leads to rapid nucleation, resulting in an increase in crystal nuclei. Tis results in a smaller size of the yielded crystal. Subsequent studies were conducted using one of these microcrystals (dropping rate 0.33 cm3 s−1). Silver orthophosphate coatings on glass substrate. CMC-Na was used as a dispersion stabilizer and thickener in the preparation of silver phosphate paste. Te Ag 3PO4 obtained above, pure water, and CMC-Na were mixed in a mass ratio of 1:1:0.020–0.050, and a paste was prepared by kneading for 10 min while maintain- ing constant humidity.

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